The present invention relates to a generator for arc welding machines which has an input rectifier stage capable of absorbing a practically sinusoidal current from the AC mains power supply.
The generator according to the invention is of the electronic type.
According to the state of the art, a classic layout for a generator for arc-welding machines is the one shown in FIG. 1, which provides for the presence of a rectifier stage 1 followed by a block 2 with clamping capacitors to which an inverter block with high-frequency electronic switches 3 is connected; the inverter block drives a transformer 4 whose secondary winding supplies a rectifier stage 5 which, by means of an inductor 6 for clamping the output current, supplies a welding arc 7.
A generator of this type absorbs from the mains, during operation, a current whose waveform has a high harmonic content, such as the one shown in FIG. 2.
With a waveform of this type a low power factor is obtained, i.e., if a high useful power level in output is required, it is necessary to absorb a high RMS current from the mains.
This entail considerable problems on the mains power supply, since the high harmonic content in the absorbed current limits the power that can be delivered by the generator, and this is a severe shortcoming particularly during use at sites with low available power, such as those served by domestic services.
This arises from the fact that the high intensity of RMS current absorbed by the generator causes the activation of the thermal protection of the system even though the useful power in output is lower than that of an equivalent resistive load.
In order to obviate these problems, welding generators have been devised which have auxiliary electronic stages which allow sinusoidal absorption of current on the mains side, so as to render the generator equivalent to a resistive load.
In this manner it is possible to absorb all the available active power.
A typical diagram of a generator according to this configuration is shown in FIG. 3, which shows, with respect to the diagram of FIG. 1, the addition of a stage, designated by the reference numeral 7, which is interposed between the rectifier block and the clamping block.
This stage, known as BOOST-type PFC, comprises an inductor 71, an electronic switch 72 and a diode 73 arranged as shown.
The switch 72 is controlled by means of an appropriate block 74 which allows to render the absorption of the generator sinusoidal, as plotted in FIG. 4.
Evidently, however, the introduction of this block entails a considerable increase in the components, and therefore in the costs, of the generator.
Additionally, the increase in components causes a reduction in efficiency due to the increased losses in the semiconductor devices.
The aim of the present invention is to provide a generator for arc-welding machines which overcomes the above-cited problems, particularly as regards the optimization of operation and efficiency.
Within the scope of this aim, a primary object of the present invention is to provide a circuit for the generator which reduces the number of electronic components, in order to reduce costs and improve efficiency.
These and other objects which will become better apparent hereinafter are achieved by a welding generator comprising a current transformer with a primary winding and a secondary winding, which supplies, at the secondary winding, a welding arc, a primary winding of said transformer being supplied by an inverter block with high-frequency electronic switches preceded by a block of clamping capacitors, said capacitors being supplied by an input rectifier stage, characterized in that said input rectifier stage has a unit power factor and is composed of at least one inductor, on a mains side, and of at least two diodes and at least two electronic switches, one for each diode, said switches being driven by a driver which renders the absorption of the input rectifier stage practically sinusoidal.